Adjusting rockers with the engine running is the least accurate method and can cause serious engine damage if done incorrectly.
Adjust them according to the service manual or if it's an aftermarket cam use their recommended procedure. For an SBC or BBC a simple method is start on #1 TDCC and adjust I & E to zero lash plus 3/4 - 1 full turn. Rotate the crank 90° and repeat in the firing order #8 - 4 - 3 - 6 -5 -7 - 2
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